Chapter 15: Lactation

1. Chapter 15: Lactation • Chapter overview: • Chapter 15 reviews the anatomy and physiology of lactation, including: • comparative anatomy of mammary system of various mammals • growth and development of the mammary system • physiology of milk production and release

2. Lactation: • Lactation may be defined as milk production • A part of the definition of a mammalian species is the ability to produce milk • Mammary glands produce the milk • Mamma - a Latin word which means breast • Milk production is an endocrine and exocrine function

3. Mammary System: Comparative Anatomy • Bovine - four glands in the inguinal area; each teat serves one gland • Ovine and caprine - two glands in the inguinal area; each teat serves one gland • Equine - four glands formed into two units in the inguinal area; each teat serves two glands

4. Mammary System: Comparative Anatomy • Porcine - multiple glands from pectoral to inguinal area, arranged in two rows; each teat serves two glands • Canine and feline - multiple glands from pectoral to inguinal area, arranged in two rows: each teat has up to 10 streak canals

5. Structures of the Mammary Gland: • Alveolar cell- basic cell producing milk in the mammary gland • Alveolus - microscopic unit made up of alveolar cells with an open lumen for collection of secreted milk • Myoepithelial cell- muscle-like cell surrounding the alveolus; contracts to express milk from the alveolar lumen

6. Structures of the Mammary Gland: • Collecting duct - tubules formed into a network from fine ducts leaving each alveolus to ever enlarging branches that terminate in the udder cistern • Udder cistern - small collecting area for milk above the teat • Teat cistern - open collecting area in the teat

7. Structures of the Mammary Gland: • Teat - the terminal structure of the mammary system from which milk will be nursed or drawn by machine • Streak canal - exit from the teat surrounded by a sphincter muscle to maintain closure until nursing or milking • Udder - term applied to the entire system

8. Manufacture of Milk: • Milk is manufactured in the mammary gland by a combination of screening components from the blood and by construction of components from precursors • Blood brings components via the pudic arteries • Blood carries away the byproducts via the pudic and subcutaneous abdominal veins • Lymph system carries away escaped plasma

9. Mammary Development: • Present at birth in mammals: • Teats • Streak canals • Teat and gland cisterns • Rudimentary collecting ducts

10. Mammary Development: • Impact of puberty and cycling • Extension of duct development • Impact of pregnancy • Development of alveolar system • Generally visual evidence of growth • At parturition the mammary system of most farm mammals is fully developed for the subsequent lactation period; swine continue development until peak lactation

11. The “Dry Period”: • The “dry period” is a period of nonlactation between subsequent lactations • A dry period is necessary for optimum lifetime production • Between lactations the mammary gland proceeds through a revitalization process of involution, alveolar regeneration, and return to lactation at the next parturition

12. Hormonal Impact on Lactation: • Prolactin - initiates lactogenesis (lactation); source: anterior pituitary • Somatotropin - influences growth rate of the glands and level of milk secretion; source: anterior pituitary • Thyroid hormones - influences milk production by regulating rate of metabolic processes; source: thyroid gland

13. Hormonal Impact on Lactation: • Parathyroid hormone - regulates blood calcium and phosphorus; related to “milk fever”; source: parathyroid glands • Adrenal hormones - small amounts necessary for normal milk production; large amounts during stress are counter-productive; source: adrenal glands

14. Hormonal Impact on Lactation: • Oxytocin - causes contraction of myoepithelial cells for milk letdown; source: posterior pituitary • Placental lactogens - hormones with prolactin and growth hormone-like activities; source: placenta • Insulin - moves glucose across membranes for milk synthesis; source: pancreas

15. Milk Synthesis: • Water, vitamins and minerals - screened from arterial blood • Lactose - manufactured in the alveolar cell • Protein - 90 percent is manufactured in the alveolar cells; some passage from blood; critical exception: immunoglobulins in colostrum are transported from dam’s blood • Fat - 75 percent is manufactured in alveolar cell

16. Milk “Letdown”: • Milk letdown is the evacuation of milk from the alveolar lumen to the duct system • Nerves receive stimuli at the teat end or nipple • Stimuli is received in the posterior pituitary • Oxytocin is released from pituitary into blood • Blood circulation brings oxytocin to mammary tissue • Targets are the myoepithelial cells that contract

17. Normal Cessation of Production: • As a lactation cycle proceeds, alveolar cells are lost normally • Eventually an animal ceases milk production due to degree of loss of secretory cells • “Drying off” and involution can be enhanced by cessation of milking or nursing and reduction of caloric intake

18. Factors Impacting Milk Production: • Inheritance - genetic selection for increased production is effective • Stage of lactation - cows “peak” about 45 days into lactation, then decline until dry • Frequency of milking - more milk is harvested with more frequent milking

19. Factors Impacting Milk Production: • Pregnancy - late pregnancy fetal demands reduce production, but pregnancy is required for normal initiation of lactation • Age - production increases as animals reach maturity, but decreases as older animals begin to physically decline • Estrus - production is generally reduced during estrus

20. Factors Impacting Milk Production: • Dry period - lack of a dry period will reduce production in subsequent lactation • Body condition - over or under conditioned animals may have reduced production • Environment - high temperature and humidity reduce milk production; extreme cold stress reduces milk production

21. Factors Impacting Milk Production: • Feed - well balanced diets are critical to maximum production and proportion of some components in milk • Proper preparation at milking time and lack of stress - critical to milk letdown and complete removal of milk